Type II Germanium Clathrates from Zintl Phase Precursor Na4Ge4: Understanding Desodiation Processes and Sodium Migration Using First-Principles Calculations

Type II germanium clathrates have recently been investigated for potential applications as anodes in batteries due to their cage-like structures that can accommodate electrochemical insertion of guest ions. To synthesize type II Ge clathrates (Ge136), several experimental routes use thermal or electrochemical desodiation of the Zintl phase compound Na4Ge4. However, the mechanism by which Na atoms are removed from the precursor to form clathrates is not well understood. Herein, we use first-principles density functional theory and nudged elastic band calculations to understand the reaction mechanism and formation energies of the products typically observed in the synthesis, namely, NaδGe136 (0 1–xGe3+z. Specifically, we confirm the energetic feasibility of Na vacancy formation in Na4Ge4 and find that the barrier for Na vacancy migration is only 0.37 eV. This relatively low energy barrier is consistent with the ease with which Na4Ge4 can be desodiated to form the products. We also discuss the energetics, sodium migration pathways, and potential electrochemical performance of Ge136 as anode material for Na-ion batteries. Overall, this study highlights how first-principles calculations can be used to understand the synthesis mechanism and desodiation processes in clathrate materials and will help guide researchers in the design and evaluation of new open framework compounds as viable materials for energy storage applications.